ESR spectroscopy of the paramagnetic complexes on DNA fibers affords simple assessment of the stereospecificity of the binding. However, the paramagnetic complexes investigated so far were mostly the stable ones in an aerobic condition, although various bioreactive metal complexes are unstable in that condition. We investigated EPR spectra of DNA fibers containing a nitrosyl iron(II) complex of cationic water-soluble porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridino)porphyrin (TMpyP) prepared in an anaerobic condition. Herein we studied the orientation of NO on the porphyrin bound to DNA. [unreadable]